1. Field of the Invention
The present invention is generally related to signal interrogation devices and, more particularly, to an apparatus for receiving a plurality of input signal pulses and determining whether the input signal pulses are of an appropriate frequency for receipt of a signal representing light pulses directed toward a photodetector apparatus.
2. Description of the Prior Art
Many types of photodetectors are known to those skilled in the art. For example, British patent 1,292,553, which Was published on Oct. 11, 1972, discloses an optical sensing system that is useable in ambient light. The disclosed device utilizes ultraviolet light and an associated photodetector which is sensitive to ultraviolet light. Both the light source and the photodetector are positioned on one side of a conveyor belt and another ultraviolet light source and associated photodetector means are positioned on the other side of the belt so that the label of a container will be exposed to the light source and sensed irrespective of its position. This British patent is one example of a photodetector system that utilizes pulsing light to permit the circuitry to distinguish proper between light signals emanating from the light source and ambient light or light from inappropriate light sources.
U.S. Pat. No. 4,649,270, which issued to Goldenberg on Mar. 10, 1987, discloses a photoelectric object detector which has a removable field stop means. This patent discloses a pulsed LED photoelectric control apparatus which operates in both a proximity mode and the retro-reflective mode. Field stop elements are manually inserted for operation in the retro-reflective mode and are quickly and readily removed for operation in the proximity mode in order to enable the same photoelectric control apparatus to be utilized in both modes. This patent no only describes the concept of pulsing light to a light sensitive device, such as a photoelectric object detector, but also illustrates one of the several arrangements that can be used to detect objects. Some applications utilizes a retro-reflector target that causes light to reflect back toward a photo sensitive apparatus after being provided by a light source which is located on the same side of the detected object as the light sensitive apparatus.
U.S. Pat. No. 4,634,855, which issued to Friend et al on Jan. 6, 1987, discloses a photoelectric article sensor with facing reflectors. The device provides a sensor for counting articles, such as seeds, which are moving through a conduit. It includes an array of infra-red LED's generating a uniform diffuse beam of radiation which entirely illuminates a cross sectional volume of the conduit. The beam is detected by a planer photo diode which extends across the opposite side of the conduit. A pair of mirrors extend along opposite sides of the conduit between the array and the photo diodes.
U.S. Pat. No. 4,633,081, which issued to Hiramatu on Dec. 30, 1986, discloses a photoelectric switch which comprises a light projecting part, a light receiving part and a control part for controlling these parts. The light projecting part and the light receiving part are fixed in mounting holes. This patent is an example of a photoelectric arrangement in which the light source is disposed on the opposite side of the detectable object from the light receiving apparatus.
U.S. Pat. No. 4,516,020, which issued to Simpson et al on May 7, 1985, describes a light- operated proximity detector with a linear output. Reflected light from a surface whose proximity to the detector is to be gauged is translated directly into a signal proportional to the distance of the detector from the surface. A phototransistor is used to sense the reflected light and is connected in a detector circuit which maintains the phototransistor in a saturated state. A negative feedback arrangement using an operational amplifier connected between the collector and emitter of the transistor provides an output at the output of the amplifier which is linearly proportional to the proximity of the surface to the detector containing the transistor.
U.S. Pat. No. 4,381,446, which issued to Fukuyama et al on Apr. 26, 1983, describes a photoelectric switch which comprises a light projecting segment that includes a pulse oscillator and a light emitting element adapted to give a pulse light emission in response to an output pulse of the pulse oscillator. It has a light reception segment which includes a light reception element that is adapted to yield a light reception signal on incidence of light. Furthermore, it provides a gate circuit that is adapted to gate the light reception signal according to the output pulse of the pulse oscillator and an integration circuit for integrating outputs of the gate circuit. It also provides a control circuit for varying the pulse frequency by controlling the pulse oscillator on generation of a light reception signal from the light reception element.
In certain applications of photodetectors, the light source is disposed on one side of an object to be detected and a light sensitive apparatus is disposed on the opposite side of an object which is to be detected. If the particular application permits, a signal line can be extended between the light source and the detector s that the light pulses emanating from the light source can be synchronized precisely with the circuitry of the detector so that the light sensitive apparatus can accurately and definitely determine whether the pulses received ar coincident with the pulses sent by the light source. In this way, extraneous light pulses can be ignored and the combination of the light source and the photodetector can accurately and positively determine whether or not an object is disposed between them. Certain applications, however, restrict the use of a signal line between the light source and the photodetector. Therefore, the photodetector sometimes can not be provided with a definite indication of the transmission of light pulses from the light source. Some means must be used to determine whether the received light pulses are being transmitted from the light source or are extraneous.
Known systems typically receive the light pulses, amplify them and compare the magnitude of the incoming light pulses to a minimum threshold level. If the incoming light pulses exceed the magnitude of the threshold, they are provided at an input of a one-shot which, in turn, provides an output pulse of a specified duration. That output pulse is then sent to a capacitive circuit which utilizes the energy received from the one-shot to partially charge a capacitor. After the termination of the one-shot output, the capacitor begins to discharge at a predefined rate which is dependent on the circuit components. As additional subsequent pulses are received, amplified and provided to the one-shot, the capacitor is sequentially provided with additional partial charges. The circuit components are selected to permit the capacitor to be sequentially charged, in steps, at a rate which is faster than the ability of the capacitor to discharge between received pulses. After the capacitor receives a sufficient number of step charges from the one-shot, a device is used to signify the fact that the magnitude of charge on the capacitor has exceeded a predefined threshold. This device can be an apparatus such as a Schmitt trigger. When an output from the Schmitt trigger occurs, it provides an indication that a sufficient number of light pulses have been received to indicate that the light pulses are of the appropriate frequency and number to indicate, with a high degree of probability, that the light pulses are being sent by the light source associated with the photodetector.
As can be imagined, a device such as that described immediately above requires a predetermined number of consecutive pulses before the capacitor charge is sufficient to cause an output from the Schmitt trigger. This is necessary to make sure that the pulses aren't random and extraneous but, instead, are pulses provided by the appropriate light source. In most applications known to those skilled in the art, a significant number of pulses must be received before the circuit determines that the light pulses are from an appropriate light source. These known applications require a significantly large number of received pulses, as described above, because they do not discriminate upon each individual pulse whether that particular pulse in an appropriate pulse or merely the result of noise. This lack of discrimination necessitates a larger number of pulses to be received before the system can be sufficiently confident that the series of pulses actually emanated from the expected source. In some applications, the required number of light pulses exceeds thirty before the circuit provides a signal indicating the receipt of pulses of an appropriate frequency. It should be clearly understood that extraneous pulses received by devices such as photodetectors are not always extraneous light pulses but, instead, are very frequently caused by electromagnetic interference (EMI) or radio frequency interference (RFI) pulses. Therefore, when a photodetector is monitoring incoming signal pulses caused by light emanating from another device, extraneous pulses can be received either by other sources of light or from sources of electromagnetic or radio frequency interference. In fact, in many applications, the electronic interference is more likely to present a problem than the receipt of extraneous light pulses from the surroundings of the photodetector. The reduction in the number of consecutive pulses necessary to discriminate appropriate light pulses from noise can be achieved if a system is developed which is significantly more discriminatory during the receipt of signals. That type of improved system could significantly increase the speed by which a photodetector determines the appropriateness of a series of light pulses under conditions which are similar to that which would require many more pulses for devices known in the prior art to make that same determination. In certain applications, the time necessary to receive the predetermined number of pulses is too great to permit the system to work properly. For example, if objects are rapidly moving through the sensing zone, the system may not have sufficient time to resynchronize during the brief period of time when light is received by the photodetector between the passing of the objects through the sensing zone. A system which reduces the number of consecutive pulses that must be received before a photodetector can confidently determine that the light pulses are from the appropriate light source would permit photodetector systems to work more rapidly and without the need for a large number of light pulses to be received before the detection circuit can confidently decide that the light pulses are proper and appropriate.